1. Field of the Invention
The present invention relates to a phase shift mask and a fabricating method thereof.
2. Description of the Related Art
As the need for thin and compact size semiconductor devices increases, semiconductor devices having larger capacities have been widely studied. To improve the resolution of such devices, various photolithography processes are intensely studied, especially those involving mask processes. Among these mask processes, phase shift mask fabricating technology, which is capable of patterning a device using a phase transition effect to achieve a high resolution, draws much attention from the public and industry. Among the phase shift mask types used in phase shift mask fabrication, the alternating type, the rim type, and the attenuating type are well known in the industry. In each of these types, a pattern of a phase transition layer can be formed beside the pattern of a light shielding layer. This is in contrast to other types of phase shift masks that use a pattern of light shielding layer formed on the light transmitting substrate.
The phase shift mask is directed to provide an additional pattern of a phase transition layer on a substrate, whose pattern has a phase difference angle of 180.degree. with respect to the phase of light passing through a substrate having another, better light transmitting characteristic. The light transmitting rate of a phase transition layer is present in various ranges. In addition, the phase shift masks of an alternating type and of a rim type (but not of an attenuating type) have an appropriately arranged light transmitting region and phase transition layer at the boundary of the light shielding layer.
The alternating type phase shift mask has a light transmitting region and a phase transition layer which are alternatingly arranged to the light shielding layer, and some patterns are additionally formed at the time of fabricating a device, so that the light intensity is increased. The rim type phase shift mask is basically directed to form a pattern of a phase transition layer, having relatively greater width than that of the pattern of the light shielding layer, on the pattern of the light shielding layer. The attenuating type phase shift mask is directed to use a light transmitting layer which does not include a light shielding layer and a layer of a phase transition having a relatively low light transmitting rate. Moreover, it is directed to use entire regions, except the light transmitting region as a phase transition layer.
The above described phase shift masks are difficult to use in the industry because there are many problems in the fabrication and maintenance process of a reticle, such as a pollution of a substrate caused during the fabrication of a semiconductor device, and in the design and test of the reticle. It is particularly difficult to use a rim type phase shift mask in the industry.
As shown in FIG. 1A, a phase shift mask according to the conventional art includes a light shielding layer 2 of chrome formed on a light transmitting substrate 1, a phase transition layer 3 formed on the light shielding layer 2, and a light transmitting poly-methyl-methacrylate ("PMMA") formed on the phase transition layer 3 with a thickness T. Thereafter, a photosensitive film 4 is formed on the phase transition layer 3, and the phase transition layer 3 of the region not masked by the photosensitive film 4 is removed so that a pattern of the phase transition layer 3 is exposed.
As shown in FIG. 1B, after the photosensitive film 4 is removed, the light transmitting layer 2 not masked by the pattern of the phase transition layer 3 is etched by a wet-type etching method. At this time, because the light shielding layer 2 is undercut, the width of lower portion of the pattern of the light shielding layer 2 is smaller than that of the phase transition layer 3. Thus the substrate 1 is exposed by the width W from each edge of both sides of the phase transition layer 3. Therefore, the light consecutively passed through the above described edge portions of the phase transition layer 3 and the substrate 1 and the light passed through only the substrate 1 are shifted by the angle of 180.degree. from each other.
A fabricating method of a conventional rim-type phase transition mask will now be explained.
First, as shown in FIG. 2A, a light shielding layer 12 of chrome is deposied on a light transmitting substrate 11. Thereafter, after a pattern of a photosensitive film 14 is formed on the light shielding layer 12, the light shielding layer 12 of the region unmasked by the pattern of the photosensitive film 14 is etched to expose the pattern of the light shielding layer 12 under the pattern of the photosensitive film 14.
As shown in FIG. 2B, a phase transition layer 15 of PMMA, which is a light transmitting photosensitive film, is coated on a region of the pattern of the light shielding layer 12 and the substrate 11. Thereafter, when light from a light exposing apparatus (not shown) is exposed to the rear side surface of the substrate 11, the region of the phase transition layer 15 of the pattern of the light shielding layer 12 is shielded from the light; however, the region of the phase transition layer 15 of the substrate 11 is exposed to the light.
As shown in FIG. 2C, the phase transition layer 15 is photoetched so that the region of the light shielding layer 12 has a pattern of the phase transition layer 15. At this time, the thickness T of the pattern of the phase transition layer 15 is such that it causes a phase shift.
As shown in FIG. 2D, the pattern of the light shielding layer 12 of the region not masked by the pattern of the phase transition layer 15 is etched by a wet-type etching method. At this time, the pattern of the light shielding layer 12 is undercut, and the width of the lower portion of the pattern of the light shielding layer 12 is smaller than that of the pattern of the phase transition layer 15, and each edge, having a width W, of both sides of the substrate of the pattern of the phase transition layer 15 is exposed. Therefore, light consecutively passed through the above described edge of the phase transition layer 15 and the substrate 11, in order, and light passed through only the substrate 11 are shifted by 180.degree. from each other.
Because the conventional phase shift mask fabricating method is directed to each the region of the light shielding layer except the region of the pattern of the light shielding layer on the light transmitting substrate, the surface of the light transmitting substrate can be damaged. In addition, because the conventional phase shift mask fabricating method involves wet-etching the region of the light shielding layer not masked by the pattern of the phase transition layer and forming the pattern of the light shielding layer in which the lower portion of the pattern of the phase transition layer is undercut, it is difficult to provide a phase transition layer having a thickness and a width from each edge of both sides of the phase transition layer, such that a desired phase shift effect can not be obtained.